I oversimplified for the sake of brevity, at the cost of accuracy: I said that the Great Red Spot is in essence a hurricane, without explaining why it resembles terrestrial hurricanes, but is still a very different kind of beast.

This post is my penance, though in my defense I was waiting for Hurricane Irene to hit and had a few other things on my mind. (On the other hand, I won’t apologize for calling Hurricane Irene a pansy.)

Atmospheres are messy, complicated affairs, which is why I study simpler things like general relativity and the universe as a whole. However, even someone like me can understand the basic principles governing planetary weather, even if I risk committing Sin #2 again. The basis for comprehension is along the same line as the earlier post on centrifugal forces: planets rotate on their axes, and this rotation produces another type of force known as the Coriolis force. This one is a little trickier mathematically than the centrifugal force, but still conceptually straightforward.

Points at different latitudes on Earth will move at different speeds, since every point must rotate around the axis in 24 hours.

The figure shows Earth (for the moment — I’ll get to Jupiter shortly), with lines of equal latitude marked with the same colors. Every point on Earth completes one rotation about the axis in the same amount of time — roughly 24 hours. This means a point on the Equator (blue line) has to move faster than a point nearer the pole (red lines). Imagine then shooting a cannon due north from the green line in the northern hemisphere; as the cannonball flies north, the surface keeps moving under it, so it actually lands at a point northeast of its starting position. (Remember Earth rotates from west to east, which is why the Sun rises in the east and sets in the west.) How far east it lands depends on how fast it was traveling, and of course what the original latitude was — the effect will be stronger closer to the equator. If you fire the cannon due south from the green line, the cannonball might actually land southwest of where you fired it, since moving into lower latitudes brings it to where the surface is rotating faster than at its starting point. The path of the cannonball curves as though a force is acting on it — the Coriolis force. This effect is opposite in the southern hemisphere.

The Coriolis force and its effect on projectiles (left side) and air flows (right side). Note that things are opposite in the southern hemisphere compared with the northern.

For ordinary everyday projectiles like baseballs, the Coriolis force isn’t very large: the distances and speeds involved aren’t big enough to get a measurable deviation. (It’s also too small to affect toilets and bathroom sinks.) However, when the “object” is a mass of air moving at appreciable velocities, the Coriolis force can be very large, and produces rotations. The rotation is counterclockwise in the northern hemisphere and clockwise in the southern hemisphere, and is known as cyclonic. The specific behavior of the rotation of air masses is complicated by oceans, which add moisture to the air and act as a giant heat reservoir, and by landmasses, which can add dry air to the mix and change the way air flows. This is why hurricanes form over water and become disrupted (eventually) when they make landfall.

Jupiter is simultaneously simpler and more complicated. First of all, Jupiter has no surface to speak of: the atmosphere blends smoothly into the interior as the pressure grows higher. Therefore, even though Jupiter has a liquid interior of hydrogen and helium, it’s not an ocean: there’s no crust beneath, no floor, nothing solid until the very core (according to the best models). This means first that the amount of time for Jupiter to make one rotation around its axis isn’t the same everywhere: the “day” at the equator is about 5 minutes shorter than the “day” at the poles. In addition, one full rotation on Jupiter is about 10 hours, so because the planet is a lot larger than Earth, its rotation speed is a lot larger… and its atmosphere is consequently a lot more turbulent, with larger Coriolis forces at work.

Because there is no land or ocean, Jupiter’s atmosphere separates into bands along equal latitudes, which are strongly colored. (Why they are colored the way they are is still a mystery!) Alternating bands of weather move in opposite directions relative to Jupiter’s rotation. Apparently because of this structure, storms don’t drift very far north or south the way they do on Earth: they tend to stick to the latitude where they formed, and they can last a long time.

A cartoon depiction of the Great Red Spot (adapted from Carroll & Ostlie's astrophysics textbook). Arrows indicate direction of atmospheric flow; note that the Red Spot sits at the boundary between bands moving in opposite directions.

Now we finally can talk about the Great Red Spot: a huge hurricane-like storm that has persisted over 300 years. It lies at the boundary between two bands, which drive the direction of the rotation. Because Jovian wind speeds are a lot higher than on Earth, the Red Spot is a far more violent storm than anything possible on Earth. It doesn’t stay at the same longitude — it rotates around Jupiter’s axis about once every 14 Jovian days (6 Earth days). Because of the flow in the bands, the Red Spot’s rotation is actually opposite to what the Coriolis force produces, so it is known as an anti-cyclonic storm. (Anti-cyclonic storms also occur on Earth, again because of the details of atmospheric interactions.)

Seasonal variation on Earth obviously plays a big role in hurricanes: there are no northern hemisphere hurricanes in January, or southern hemisphere hurricanes in August. Jupiter lacks strong seasons: its axis is pretty much straight up-and-down, so each side of the planet gets the same amount of sunlight all year. Saturn is similar to Jupiter in many respects, but has a larger axial tilt than Earth, so it experiences large seasonal variations in its storms — which can be as violent as Jupiter’s, though none have been seen that are as persistent as the Great Red Spot.

This much is uncontroversial (I think — planetary scientists please correct me if I’m wrong on this), but there’s a lot astronomers don’t understand yet. The red color is mysterious — it’s not iron like on Mars, but it may be an organic (carbon-containing) or a sulfur compound. The “eye” of the storm is slightly warmer than the outer regions, hinting that there may be some convection from the hotter lower atmosphere or perhaps the interior. Nobody is sure how long the storm has lasted — it was first discovered by astronomer Giovanni Cassini in the late 1600s — or how long it will continue to exist. Without a more complete model of how the Red Spot works, including flows of heat throughout Jupiter’s atmosphere, we can’t be entirely sure.

Hurricanes like Irene are natural consequences of the flow of heat energy through the atmosphere and oceans of Earth, and are shaped by the Coriolis force. Storms like Jupiter’s Great Red Spot (and perhaps Neptune’s Great Dark Spot) are consequences of the even more violent wind patterns and lack of land or oceans. The dynamics at their hearts are very alike, but as we often find, the contrasts can tell us as much as their similarities.